The automobile industry is required to reduce fuel consumption through weight reduction and demand for high-strength steel plates has been increased to achieve weight reduction.
The strength of steel plates has been improved by precipitation strengthening that precipitates carbides in a steel, a solution strengthening that adds Si and Mn to a steel or strengthening that produces low-temperature transformation products.
When carbon content is increased for precipitation strengthening by carbides, weldability deteriorates in some cases. When a large amount of alloy elements, such as Si and Mn, is added, chemical conversion treatability deteriorates or manufacturing cost increases in some cases. When a large amount of alloy elements is added, the strength of steel plates increases excessively during hot rolling and cold rolling requiring higher rolling force and it is difficult to manufacture steel plates of a desired size (thickness and width).
Technique proposed in Patent document 1 reduces the amount of alloy elements contained in a steel plate to reduce the strength so that the steel plate can be hot rolled or cold rolled without increasing rolling force and enhances the strength by precipitating Ti nitride produced by nitriding Ti contained in the steel by annealing after rolling. However, the control of an atmosphere for a nitriding process is difficult. If N is dissolved excessively in a steel, blowholes are formed in the steel during welding, the strength of a welded joint is reduced and weldability is deteriorated. Patent document 1 cools a coil of a nitrided steel directly to an ordinary temperature and hence an excessively large amount of N is dissolved in the steel. Nitrogen contained in the steel produces blowholes during welding and hence the weldability of the steel is unsatisfactory.
Patent Document 1: JP-A 2001-507080